This invention relates in general to microwave transitions, and more particularly, to transitions from a microstrip transmission line medium to a coplanar waveguide transmission line medium.
Microwave modules often combine discrete component circuits with microwave monolithic integrated circuits (MMICs). Microwave modules are also typically designed using microstrip transmission line as the preferred transmission line medium because of its structural simplicity, and because microstrip facilitates engineering and manufacturing operations such as assembly, tuning, and rework.
Integrating MMICs into such a module often does not require transitions from the microstrip in the module to the inputs and outputs of the MMICs at higher microwave frequencies. Typically, the more expensive MMICs designed to operate at these frequencies use microstrip, and wire bonding between the top conductors of the module microstrip and the MMIC microstrip.
MMICs are often the most expensive components in a module assembly and techniques for reducing their cost have been an important priority in their continued development. Many lower cost MMIC components which are commercially available, or designed for low cost fabrication processes rely on coplanar waveguide as a transmission line medium. Coplanar waveguide permits fabrication of MMIC transmission lines without the need for expensive or yield limiting process steps such as wafer thinning, and backside processing.
The use of coplanar waveguide MMICs in modules using microstrip design requires some low loss means of transitioning from microstrip in the module to coplanar waveguide on the MMIC. Such a means when reversed can transition from coplanar waveguide on the MMIC to microstrip in the module so that lower cost coplanar waveguide MMICs can be integrated into a microstrip module assembly.
Lower frequency transition techniques typically employ bondwires to connect the microstrip top conductor with the coplanar waveguide center strip directly from where the module substrate ends to where the MMIC substrate begins. Bondwires are also used to connect the coplanar waveguide outer conductor strips to the grounded module base plate. The module base plate also grounds the backside ground plane of the microstrip. The signal carrying conductors of both transmission lines are connected together with bondwires, as are the ground conductors of both transmission line types. At higher frequencies, the bondwire inductive reactances, become excessively high. Impedance mismatch due to high inductive reactances cause signal reflections which degrade microwave performance.
High frequency transitions from coplanar waveguide to microstrip exist in the form of coplanar waveguide test probes which are used in conjunction with structures on a microstrip MMIC to form a transition. Coplanar waveguide test probe transitions are not useful for microstrip modules in transitioning to coplanar waveguide MMICs.
Since newer MMIC processes are combining higher frequency devices with lower cost coplanar waveguide design, a high frequency transition from module microstrip to MMIC coplanar waveguide is required to enable the use of these lower cost MMICs in microstrip modules at higher frequencies. Such a high frequency transition should be well matched, so as to minimize mismatch loss, and easily produced with conventional assembly techniques.